1. Field of the Invention
The present invention relates to an apochromatic lens system for use as an objective lens system in, for example, an astronomical telescope.
2. Description of the Related Art
It has been known that an objective lens system of an astronomical telescope has a combination of a single positive lens element and a single negative lens element, which are made from lens materials having different dispersion powers. Since the objective lens system of an astronomical telescope has a long focal length, correction of axial chromatic aberration is especially important; in recent years there have been many examples of fluorite or special low-dispersion glass being used to reduce the secondary spectrum of the axial chromatic aberrations.
As examples of objective lens systems with a large diameter, the astronomical telescopes disclosed in Japanese Laid Open Patent Publication No.57-93309 and in Japanese Laid Open Patent Publication No.59-220711 have been known. In these examples, a first lens group is composed of a combination of positive and negative lens elements made from lens materials having different dispersion power, and a second lens group distant from the first lens group toward the image is provided; correction of axial aberrations such as chromatic aberration and spherical aberration are carried out through the first lens group, and correction of field curvatures is carried out through the second lens group. Due to the above explained lens arrangement, the lens system having both a large diameter and a wide angle-of-view is materialized.
However, in both of the above-referenced astronomical telescope objective lens systems, an expensive material, such as fluorite or special low-dispersion glass, has had to be employed to make the large-diameter positive lens element in the first lens group in order to reduce the secondary spectrum of axial chromatic aberrations. These factors have resulted in an increase of the overall production costs.